Recently, a flat-type X-ray detector using active matrix is attracting attention as a new-generation X-ray detector for diagnosis. The flat-type X-ray detector is configured to output an X-ray image formed by X-rays or a real-time X-ray fluoroscopic image as a digital signal.
The flat-type X-ray detector is a solid detector, so that it is highly expected in terms of the improvement and stability of performance.
The flat-type X-ray detector for general radiography and chest radiography to collect static images with a relatively large radiation dose has been developed and become commercially available. It is also possible to detect real-time X-ray dynamic images of 30 frames or more per second with a relatively small dose for fluoroscopy. Therefore, products applied to the field of diagnosis of circulatory organs and digestive organs are expected to be commercialized in near future. The commercialization of the X-ray detector for dynamic images requires improvement of S/N ratio and further improvement of a real-time micro-signal processing technology.
The flat-type X-ray detector is roughly categorized into two types of systems, a direct conversion system and an indirect conversion system.
The direct conversion system uses a photoconductive layer of a-Se or the like to directly convert X-rays into electric charge, and the electric charge is collected with a pixel electrode and stored in an storage capacitor. According to this system, image resolution characteristics are superior and substantially determined with the pixel pitch. The indirect conversion system converts X-rays once into visible light by a scintillation layer, converts the visible light into electric charge by a photoelectric converting element such as an a-Si photodiode or CCD, and the electric charge is stored in an storage capacitor.
In order to absorb X-ray sufficiently, the flat X-ray detector of the direct conversion system adopts for example, a photoconductive layer of a-Se to have a thickness of about 1 mm. For example, an intense bias electric field of 10 V/μm is applied to both ends of a photoconductive layer of a-Se to increase charge generation rate per X-ray photon, to collect the photoconduction charge to a electrode without being trapped frequently by defects in the layer and to suppress the electric charge diffusion in a direction perpendicular to the bias electric field as much as possible. Therefore, a high voltage of, for example, about 10 kV is applied when the a-Se photoconductive layer has a thickness of 1.0 mm.
The X-ray detector of the direct conversion system has an advantage that its resolution characteristics are outstanding, but its reliability is doubtful because TFT having a low operation voltage must be protected from a high voltage. And, there is also a disadvantage that photoconductive material having a low dark current characteristic, a high sensitivity characteristic, thermal stability and the like cannot be obtained easily.
Meanwhile, the X-ray detector of the indirect conversion system uses a photodiode, a CCD or the like to produce a signal electric charge, so that it is not necessary to apply a high voltage as by the direct conversion system, and there is no problem of dielectric breakdown by a high voltage. There is also an advantage of easy commercialization because the basic technology of the scintillator material and photodiode has been established.
But, it has a disadvantage that its resolution characteristic for X-ray diagnostic systems is generally inferior to the direct conversion system because fluorescent light converted by the scintillator layer causes diffusion or scattering before reaching the photoelectric converting element. Especially, when the scintillator layer is formed thick in order to improve the sensitivity characteristic, the resolution is degraded considerably because the diffusion of fluorescent light becomes large before reaching the photoelectric converting element such as a photodiode. To secure the resolution by suppressing the diffusion of the fluorescent light, there is a known method of forming the scintillator layer in pixel unit along with the matrix of photodiodes and TFT and optically shielding the scintillator pixels from each other by the partition between pixels. But, the partition of the conventional X-ray detector is formed of metal or the like and not contributing to the X-ray sensitivity. Therefore, it has a problem that a total light emission amount of the scintillator layer decreases by a portion replaced from the scintillator layer to the partition, and the sensitivity of the X-ray detector deteriorates.